Beverages prepared by fermenting beverages containing protein such as milk or soybean milk with lactic acid bacteria, or beverages prepared by directly adding an organic acid to beverages containing these protein are called acidic protein beverages. These acidic protein beverages are favored as beverages having refreshing sourness, but under acidic conditions, particularly under conditions that are more acidic than the vicinity of the isoelectric point of the protein, the protein aggregates and precipitates, and their value as commercial products is significantly impaired. Then, with an aim of dispersion stabilization of protein, dispersion stabilizers, such as high methoxyl pectin and carboxymethylcellulose have been used (Patent Literature 1). These conventional dispersion stabilizers are considered to electrostatically react with protein particles, as well as to form a weak network of polysaccharides in a beverage to suppress aggregate precipitation of protein by imparting viscosity, and had a problem of imparting characteristic viscosity to the beverage (Non Patent Literature 1).
In contrast, water-soluble soybean polysaccharides made by extracting polysaccharides contained in cotyledons of soybean seeds, which have a function to suppress aggregate precipitation of protein under acidic conditions like high methoxy pectin and carboxymethylcellulose, have been used as stabilizers for acidic protein beverages (Patent Literature 2). Water-soluble soybean polysaccharides electrostatically react with protein particles, suppress their aggregation by covering the surfaces of the protein particles, and stabilize the protein particles by fining the protein particles using homogenization treatment to an extent in which the particles can be dispersed by Brownian movement. Thus, the polysaccharides provide acidic milk-based beverages with a refreshing light taste, without imparting viscosity (Non Patent Literature 2).
Incidentally, acidic protein beverages can be made as beverages in various pHs that are at the vicinity of the isoelectric point of the protein or lower. In the range of pH 4.2 to pH 4.5 at the vicinity of the isoelectric point of milk protein among these, the surface charge of the protein is in a non-charged to slightly positive-charged state, and strongly negative-charged polysaccharides show a good dispersion stabilizing ability. Thus, high methoxy pectin, carboxymethylcellulose, and polymeric water-soluble soybean polysaccharides show good dispersion stability (Patent Literature 3). In contrast, under more acidic conditions at pHs lower than pH 4.2, the surface charge of protein is more strongly positive-charged. As for the strongly negative-charged polysaccharides described above, since a strong reaction with protein particles occurs to cause aggregate precipitation called depression, a good stable state cannot be achieved. In this pH range, weakly negative-charged water-soluble soybean polysaccharides show good dispersion stability.
In this way, in the conventional art, it was necessary to select an optimal stabilizer depending on the pH to be used. However, if a stabilizer applicable in a wide pH range can be obtained, industrial effects are very large because the stabilizer can prevent water separation from an acidic protein beverage in an overfermented state, and additionally, a plurality of products can be prepared from a single raw material by performing fermentation using various pHs as the end point. Not individual dispersion stabilizers used in a particular pH range described above, but a dispersion stabilizer that disperses and stabilizes protein under conditions from the vicinity of the isoelectric point of the protein to acidic conditions, specifically in a wide pH range from pH 3.2 to 4.8, particularly from pH 3.4 to pH 4.5 and without imparting viscosity to the beverage and that can maintain the stability of the beverage has been strongly required.